// This is Free Software under GNU Affero General Public License v >= 3.0
// without warranty, see README.md and license for details.
//
// SPDX-License-Identifier: AGPL-3.0-or-later
// License-Filename: LICENSES/AGPL-3.0.txt
//
// Copyright (C) 2018, 2019 by via donau
//   – Österreichische Wasserstraßen-Gesellschaft mbH
// Software engineering by Intevation GmbH
//
// Author(s):
//  * Sascha L. Teichmann <sascha.teichmann@intevation.de>
//  * Bernhard E. Reiter <bernhard.reiter@intevation.de>

package common

import (
	"math"
	"time"
)

const (
	// time.RFC3339 equals "simplified ISO format as defined by ECMA-262"
	//   https://tc39.github.io/ecma262/#sec-date-time-string-format
	// and "SHOULD be used in new protocols on the Internet." (RFC section 5.6)
	TimeFormat = time.RFC3339
	DateFormat = "2006-01-02"
)

var utc0 = time.Unix(0, 0)

func InterpolateTime(t1 time.Time, m1 float64, t2 time.Time, m2 float64) func(float64) time.Time {

	// f(m1) = t1
	// f(m2) = t2
	// t1 = m1*a + b <=> b = t1 - m1*a
	// t2 = m2*a + b

	// t1 - t2 = a*(m1 - m2)
	// a = (t1-t2)/(m1 - m2) for m1 != m2

	if m1 == m2 {
		t := t1.Add(t2.Sub(t1) / 2)
		return func(float64) time.Time { return t }
	}

	a := t1.Sub(t2).Seconds() / (m1 - m2)
	b := t1.Sub(utc0).Seconds() - m1*a

	return func(m float64) time.Time {
		x := m*a + b
		secs := math.Ceil(x)
		nsecs := math.Ceil((x - secs) * (999999999 + 1))
		return time.Unix(int64(secs), int64(nsecs))
	}
}